Learn Hardcore Portuguese (Portugal): O sinal na passadeira está avariado hoje. - The signal at the crosswalk is broken today.

O sinal = the signal / the (traffic) light – a specific one that both speaker and listener can identify (for example, the one at this particular crossing).
Um sinal = a signal / a (traffic) light – any one, not a specific, known one.

In this context, we’re clearly talking about the specific signal at that crosswalk, so Portuguese naturally uses o.

What does sinal mean here, and how is it different from semáforo?

In Portugal:

sinal (on its own, in a traffic context) often refers to a traffic light or traffic signal.
semáforo also means traffic light, and is very common and clear.

So you could easily say:

O sinal na passadeira está avariado hoje.
O semáforo na passadeira está avariado hoje.

Both are fine. Semáforo is more unambiguously “traffic light”, while sinal is a broader word meaning “signal / sign / signpost”, whose exact meaning depends on context.

What exactly does na mean in na passadeira?

Na is the contraction of:

em (in / on / at) + a (the, feminine singular)

So:

em + a passadeira → na passadeira = on the crosswalk / at the crosswalk

Similarly:

em + o sinal → no sinal (no, masculine singular)
em + as passadeiras → nas passadeiras (nas, feminine plural)

What does passadeira mean here, and is it specific to Portugal?

Here passadeira means pedestrian crossing, often what British English calls a zebra crossing.

In Portugal: passadeira is the normal term.
In Brazil: the common term is faixa de pedestres.

Grammar points:

passadeira is feminine: a passadeira, na passadeira
Plural: as passadeiras, nas passadeiras

Note: passadeira can also mean ironing board cover or a runner rug, but traffic context makes the meaning clear.

Why is it está avariado and not é avariado?

Portuguese distinguishes between:

estar: temporary state / condition (how something is now)
ser: permanent, essential characteristic (what something is by nature)

Here está avariado describes a temporary condition: the signal is out of order today, but normally it works.

Using é avariado would sound like saying “it is (by nature) faulty/broken”, which is not what is meant.

What does avariado mean, and when do I use it instead of estragado or partido?

Avariado usually means out of order / faulty / not working, especially for:

machines
devices
vehicles
systems

In contrast:

estragado = damaged / spoiled / ruined (food, objects, plans, etc.)
partido / partido ao meio = physically broken into pieces
(Brazilian) quebrado = also “broken” in many senses

For a traffic signal, avariado is the most natural:

O sinal está avariado. = The signal is out of order / malfunctioning.

You could say estragado, but it sounds more like it is damaged rather than simply not functioning.

Why is it avariado and not avariada?

Adjectives agree in gender and number with the noun they describe.

sinal is masculine singular: o sinal.
So the adjective must also be masculine singular: avariado.

Examples:

O sinal está avariado. (masc. sing.)
Os sinais estão avariados. (masc. plural)
A passadeira está estragada. (fem. sing.)
As passadeiras estão estragadas. (fem. plural)

Is the word order fixed? Could I say O sinal está avariado na passadeira hoje?

The word order is flexible. All of these are grammatically correct:

O sinal na passadeira está avariado hoje.
O sinal está avariado na passadeira hoje.
Hoje, o sinal na passadeira está avariado.
O sinal na passadeira hoje está avariado.

The original version:

O sinal na passadeira está avariado hoje.

puts na passadeira right after o sinal, highlighting which signal (the one at the crosswalk), and then comments on its condition (está avariado) and the time (hoje).

Different orders slightly change the focus, but not the basic meaning.

Can hoje (today) go in other places in the sentence?

Yes, hoje is quite mobile. These all sound natural:

Hoje o sinal na passadeira está avariado.
O sinal na passadeira hoje está avariado.
O sinal na passadeira está avariado hoje.

The differences are mainly about emphasis:

At the start (Hoje ...): emphasizes today.
At the end (... hoje.): sounds like an “afterthought”, but is very common in speech.

Meaning-wise, they all say the same thing.

Why is there an article in na passadeira? Could I say just em passadeira?

In this context, you need the article:

na passadeira = em + a passadeira = at/on the crosswalk

Without the article (em passadeira), it sounds ungrammatical here. Portuguese generally uses articles far more than English does, especially with concrete, specific places.

Compare:

Estou na escola. = I’m at school.
Estou no trabalho. = I’m at work.
O carro está na garagem. = The car is in the garage.

How do you pronounce O sinal na passadeira está avariado hoje in European Portuguese?

Approximate IPA (European Portuguese):

O sinal na passadeira está avariado hoje.
/u siˈnaɫ nɐ pɐsɐˈðejɾɐ ɨʃˈta ɐvɐɾiˈaðu ˈoʒ(ɨ)/

Some tips:

O = [u], like oo in food, but shorter.
Many unstressed a’s reduce to a schwa-like passadeira, avariado.
lh / nh aren’t present here, but -lh- and -nh- in other words have distinctive palatal sounds.
Final -e in hoje is often very reduced or almost silent: [ˈoʒ] or [ˈoʒɨ].

How would a Brazilian Portuguese speaker most likely say this?

A natural Brazilian version might be:

O semáforo na faixa de pedestres está quebrado hoje.
- semáforo instead of sinal
- faixa de pedestres instead of passadeira
- quebrado instead of avariado

Brazilian speakers will understand the European sentence, but their everyday word choices differ.